1. Field of the Invention
The present invention relates to an adapter that augments an existing radio receiver's performance.
2. Description of the Known Art
Adapters or converters are known that enable existing radio or television (TV) receivers effectively to increase the frequency range over which they can be tuned. Such converters must be hard-wired to an existing receiver between an antenna lead or a cable, and a corresponding radio frequency (RF) input terminal of the receiver. An example of such a converter is the familiar cable TV channel selector unit, for use with TV receivers that are not "cable ready". The cable TV converters operate to convert a selected cable channel to a certain VHF channel (e.g., TV channel 3 or 4) to which the viewer's TV receiver is preset. In the case of cable TV programs that are "scrambled", the converter may include appropriate descrambling circuitry for those viewers who pay a premium to the cable system operator.
The All Channel Receiver Act of 1962, 47 U.S.C. .sctn. 303(s), requires television receivers sold in the United States to include UHF tuners. Until then, UHF to VHF converters were used to enable programs broadcast over the UHF TV channels (then 14 to 83) to be selected and converted to a preset VHF channel. Owners of the many existing VHF-only TV sets were then able to enjoy reception of broadcast UHF programs, by connecting the converter between a UHF TV antenna, and the VHF antenna input terminals of their TV set which was left tuned to the preset VHF channel.
In the radio broadcasting spectrum, "short wave" (SW) converters allow owners of standard AM radios to listen to SW radio broadcasts. A suitable shortwave receiving antenna is connected to the converter, and received SW carrier signals are converted to a frequency within the standard AM radio band. Like the cable and the UHF-VHF converters mentioned above, SW radio converters must be connected by wire or cable to an existing receiver for proper operation.
It is also known to provide apparatus that determines the channel to which an existing TV receiver is tuned, by coupling the apparatus with a local oscillator (LO) signal produced in the TV receiver and processing the detected LO signal. See U.S. Pat. No. 4,577,220 (Mar. 18, 1986); U.S. Pat. No. 4,723,302 (Feb. 2, 1988); and U.S. Pat. No. 5,289,271 (Feb. 22, 1994). The patented apparatus do not serve to augment or to enhance the performance of existing TV receivers, however.
On Jan. 8, 1993, a North American Radio Broadcast Data System Standard was adopted in the United States. The Standard, now referred to as "RDS", applies to FM sound broadcasts in the range of 87.5 MHZ to 108.0 MHZ, carrying either stereophonic or monophonic programs. See United States RDS Standard, Draft 2.0, available from the National Association of Broadcasters, Washington, D.C.
When broadcasting under the RDS Standard, a 57-kHz subcarrier is added to the main program carrier signal, and source data is digitally encoded on the subcarrier for multiple purposes. For example, program type (PTY) source data identifies the type of program transmitted, including Jazz, Classic Rock, Oldies, Country, News and Information, Children, Sports, Talk, Emergency, and the like. Persons wanting to listen only to specific types of programs can set the desired PTY codes in suitably equipped FM receivers, and the receivers will then selectively seek and play only those stations which transmit the types of programs desired to be heard. The broadcast subcarrier is amplitude modulated by a shaped and biphase coded data signal, and the subcarrier is suppressed. The modulation method may be considered a form of two-phase shift keying (PSK), with a phase deviation .+-.90 degrees. A basic clock frequency is derived by dividing the transmitted subcarrier frequency by 48, so that the basic data rate of the system is 1187.5 bits per second.
Source data at the transmitter may also include displayable text characters, referred to in the RDS Standard as Radiotext (RT). The data is differentially encoded in a manner such that when the input data level is binary zero, the output stays unchanged from the previous output bit level and, when an input binary one level occurs, the new output bit is the compliment of the previous output bit. At the receiver, data is decoded by the inverse process.
Most persons spend a good deal of time in their cars while commuting, shopping, or driving a family member from one place to another. Most all vehicle radios now in service are capable of receiving the FM broadcast band using either analog or digital tuning technology. Commercial radio broadcasters therefore have an ongoing opportunity to communicate with the general public while they travel on the road. But existing FM radios have no circuitry for decoding and displaying RDS radiotext transmissions. Also, relatively few FM broadcasters are sending RDS source data on a 57-kHz subcarrier, and receivers equipped to respond to such data are not widely available for either home or vehicle use. Further, FM broadcast stations now transmitting radiotext usually limit the text to station call letters or a popular station name.
U.S. Pat. No 5,063,610 (Nov. 5, 1991) and U.S. Pat. No. 5,214,792 (May 25, 1993) disclose broadcasting systems having supplemental data transmission and storage. For example, if an FM radio broadcaster is encoding the corresponding RDS radio text, listeners with receivers constructed according to the patents can identify and store the title of a broadcast musical piece while it is being played, or they can view and store text such as a telephone number relating to an advertisement.